U.S. Pat. Nos. 3,409,579 and 3,676,392 disclose binder compositions for aggregate mixtures such as foundry mixes for making cores, molds and other foundry shapes for casting metal. The entire contents of each of these U.S. patents are incorporated herein by reference. These binder compositions may be supplied as two-package systems comprising a resin component in one package and a hardener component in the other package. The resin component comprises an organic solvent solution of phenolic resin. The hardener component comprises a liquid polyisocyanate having at least two isocyanate groups per molecule. At the time of use, the contents of the two packages may be combined first and then mixed with the sand aggregate, or preferably the packages are sequentially admixed with sand aggregate. After a uniform distribution of the binder on the sand particles has been obtained, the resulting foundry mix is molded into the desired shape for subsequently casting a metal shape.
In U.S. Pat. No. 3,409,579, the molded shape is cured by passing a gaseous tertiary amine through it. In U.S. Pat. No. 3,676,392, curing is effected by means of a base having a pK value in the range of about 7 to about 11 as determined by a method described by D. D. Perrin in Dissociation Constants of Organic Bases in Aqueous Solution (Butterworths, London 1965). The base is introduced originally into the resin component before it is mixed with hardener, or it may be introduced as the third component of a three-package binder system comprising in separate packages the resin component, the hardener, and the base.
In both U.S. Pat. Nos. 3,409,579 and 3,676,392, the preferred phenolic resins contain benzylic ether resins along with other reaction products. Benzylic ether resins are condensation products of a phenol with an aldehyde where the phenol has the general formula: ##STR1## wherein A, B, and C are hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals, or halogen, and where the aldehyde has the general formula R'CHO wherein R' is a hydrogen or a hydrocarbon radical of 1 to 8 carbon atoms, prepared in the liquid phase at temperatures below about 130.degree. C. in the presence of catalytic concentrations of a metal ion dissolved in the reaction medium. The preparation and characterization of these resins is disclosed in greater detail in U.S. Pat. No. 3,485,797, the entire contents of which is incorporated herein by reference. The phenolic resin component of the binder composition is, as indicated above, generally employed as a solution in an organic solvent.
The second component or package of the binder composition comprises an aliphatic, cycloaliphatic, or aromatic polyisocyanate having preferably from 2 to 5 isocyanate groups. If desired, mixtures of polyisocyanates can be employed. Less preferably, isocyanate prepolymers formed by reacting excess polyisocyanate with a polyhydric alcohol, e.g., a prepolymer of toluene diisocyanate and ethylene glycol, can be employed. Suitable polyisocyanates include the aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as 4,4'-dicyclohexylmethane diisocyanate, and aromatic polyisocyanates such as 2,4- and 2,6-toluene diisocyanate, diphenylmethane diisocyanate, and dimethyl derivatives thereof. Further examples of suitable polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylylene diisocyanate, and the methyl derivatives thereof, polymethylenepolyphenyl isocyanates, chlorophenylene-2,4-diisocyanate, and the like. Although all polyisocyanates react with the phenolic resin to form a cross-linked polymer structure, the preferred polyisocyanates are aromatic polyisocyanates and particularly diphenylmethane diisocyanate, triphenylmethane triisocyanate, and mixtures thereof.
The polyisocyanate is employed in sufficient concentrations to cause the curing of the phenolic resin. In general, the polyisocyanate will be employed in a range of 10 to 500 weight percent of polyisocyanate based on the weight of the phenolic resin. Preferably, from 20 to 300 weight percent of polyisocyanate on the same basis is employed. The polyisocyanate is employed in liquid form. Liquid polyisocyanates can be employed in undiluted form. Solid or viscous polyisocyanates are employed in the form of organic solvent solutions, the solvent being present in a range of up to 80% by weight of the solution.
The bench life of an aggregate binder can be defined as the maximum permissible time delay between mixing the binder components together with an aggregate such as sand and the production of acceptable products therefrom by at least partial curing. In order to extend the bench life of the above binder systems before they are contacted with the catalytic component, various materials have been suggested. Phthaloyl chloride, acid halides, phosphorus compounds and other bench life extenders are currently being commercially employed for such purposes. Great improvements in bench life have been obtained through the use of phosphorus halides as described in copending application Ser. No. 575,208, and phosphorus based acids as described in copending application Ser. No. 599,106.
Attempts have been made in the past to use adhesive compositions similar to the foregoing binder compositions to bond together foundry shapes of the type described and associated metal molds into a composite molding assembly. As used in this specification, the term "foundry shape" means molding shapes made of aggregate foundry mixes, such as cores and molds, and molding shapes made of other materials, such as metal shells and other metal molding parts for casting metal shapes. Shapes for molding plastic materials also are intended to be included within the meaning of this term.
Such prior art uses of binder type compositions as foundry shape adhesives have encountered various problems and have resulted in a number of deficiencies. These problems and deficiencies include difficult to control gel times and cure times, difficulties in application due to the Part I component (resin) being too viscous and the Part II component (hardener) being too thin (overly fluid). Such substantial differences in viscosities between the Part I and Part II components also result in poor mixing characteristics leading to unpredictable gel and curing times. Prior art systems also were deficient in requiring mixing ratios between the Part I and Part II components other than 50:50, such as 60:40. Such unequal mixing ratios between the parts make it difficult to maintain proper curing relationships between the reactants at the time of application, which again makes for unpredictable gel and curing times and limits the types of application equipment that can be used. A further deficiency of the prior art was that it was extremely difficult to precatalyze either of the components so that the catalyst had to be added as a third component at the application site. While some precatalyzation of the resin component was possible, this precatalyzed resin generally had an unacceptable shelf-life (less than a one month) due to a lack of stability of the premixed ingredients. Other deficiencies included unworkable consistencies, foaming and other characteristics causing dimensional changes after application, low tensile strengths, resoftening with heat, deterioration of adhesive upon water absorption, and the like.
Other prior art practices include the use of relatively expensive hot melt adhesives which are prone to thermal instability (resoftening or other loss of tensile strength) when subjected to heat from the molten metals being cast or other processing operations subjecting the molding assembly to heat. Hot melt adhesives also may resoften upon core wash and over drying of the parts. Such thermal instability allows the glued parts to shift, thereby ruining the tolerances of the casted metal piece. Softening of the adhesive also may result in run out of the molten metal, which similarly may destroy the tolerances of the casted pieces. Run out is due to an inadequate adhesive seal between the molding assembly parts and also may result from an improper consistency of the applied adhesive. Another problem with hot melt adhesives is they are expensive and hazardous to handle and the equipment used for their application is subject to considerable down time and maintenance.
Prior practices also include the use of air or oven dried adhesives. This class of adhesives is slow to cure and therefore significantly limits production rates. A delay of sometimes as much as 10 to 15 hours after gluing the parts of the molding assembly together may be necessary before molten metal can be poured into such an assembly. It has long been recognized that the elimination of such time delays would significantly increase production rates. In lieu of any type of adhesive, prior art practices also include the use of metal fasteners to hold the foundry shapes together during the metal casting operation. However, such metal fasteners are expensive to provide and time consuming to apply. In addition to metal fasteners, weights attached to the molding assembly were sometimes required in order to help hold the assemblied parts together during the casting process.
Although two part adhesive pastes have been used in the past, the resin component had an extremely high viscosity (about 50,000-60,000 cps) and the isocyanate component had an extremely low viscosity (about 200-300 cps). These very great differences in viscosities caused difficulties in mixing and application of the final adhesive composition. For example, it is very difficult to feed two components with such widely varying viscosities through a common applicator gun and properly control the mixing ratios because the viscosity differences result in substantial variations in pumping pressures and flow rates. Prior art applicator systems also required auxiliary solvent flush systems to remove blockages caused by improper mixing of the hardener component and/or interim delays in adhesive application.